A traditional photovoltaic panel comprises several parallel/serial associations of photovoltaic cells and develops a direct voltage of approximately forty volts at its terminals under nominal light conditions. In a minimal facility, approximately ten panels are serially connected to produce a direct voltage, in the vicinity of 400 V, that can be exploited with a good output by an inverter to transfer the energy produced onto the network.
One advantage of the serial connection of the panels is that the connector technology is reduced to two connection terminals per panel, plus one ground terminal, which facilitates installation. The panels are thus equipped with standardized junction boxes comprising the necessary terminals.
Nevertheless, the serial connection may have a number of problems.
The current produced by a serial string of panels is determined by the weakest link, i.e. the panel generating the weakest current. That panel may simply be a panel located in the shade. In such a situation, it is necessary to establish a path short-circuiting the panel, such that the panels operating under normal conditions can throw their nominal current. To that end, the panels are equipped with so-called “bypass” diodes, connected between the terminals of the panel, in the direction of the current, which is generally the blocked direction of the diodes relative to the voltage generated by the panel. When a panel no longer generates any voltage, the current of the string passes through its bypass diodes.
However, when a panel is partially in shade, it will produce a voltage below its nominal voltage, but sufficient to avoid activating the bypass diodes.
To manage such a situation more smartly, it has been provided to equip each photovoltaic panel with a control module electrically powered by the panel, as described in U.S. Pat. No. 7,602,080.
FIG. 1 diagrammatically illustrates a local control module 10 (LCU) associated with a panel 12, as described in the aforementioned patent. The LCU control module is connected to the panel 12 by two connection terminals A1 and A2, terminal A1 being connected to the “+” of the panel, and terminal A2 to the “−”. The module includes two terminals B1 and B2 for connecting it in series by a single conductor 13 to homologous modules. The cathode of a bypass diode D1 is connected to the terminal B1 and the anode of said bypass diode is connected to the terminal B2. The direction of the serial current in the conductor 13 is thus from the terminal B2 toward the terminal B1. A switch S, controlled by a circuit 14, is connected between the terminals A1 and B1. A capacitor C1 is connected between the terminals A1 and A2.
The control circuit 14 is powered by the panel 12, between the terminals A1 and A2. It communicates with a shared central control unit located at the inverter through a COM link. To avoid multiplying the number of connections between panels, this link may be done by carrier current on the serial link conductor or by wireless communication.
The purpose of this management system is to control, in switching mode, the switch S of a module associated with a lowly-lit panel to optimize the energy transfer.
As indicated, the LCU control modules are powered by the associated panel 12. If the electricity production of the panel is insufficient, the module no longer works. In that case, the module is unable to communicate with the central control unit, in particular to indicate the permanent or temporary out-of-service status of the panel.
The system described in the aforementioned patent uses complex communication means between the modules and the central control unit. Each module must incorporate a microcontroller and a modem by carrier current or by wireless communication. These means are too costly for bottom-of-the-line facilities into which one nevertheless wishes to integrate certain basic functions.
A fleet of photovoltaic panels has a risk of electrocution during assembly. In fact, a lit panel, even a disconnected one, begins to produce electricity. As the panels are connected in series, the difference in potential between the end terminals of the mounted panels increases, that potential difference reaching the vicinity of 400 V when it is time to connect the last panel.
In current fleets, it is difficult to locate the site of an accidental cut in the serial link conductor. In fact, the cut of the serial conductor cancels the current therein. All the units of the panels see cancellation of the current at the same time, such that a module, even a smart module, cannot determine that the cut has occurred at its level to indicate that fact.